The invention concerns a method for the detection of not easily volatized substances, in particular of Nitrotoluene, trinitrotoluene, dinitrotoluene or derivatives of nitrotoluene as well as chemical warfare materials such as Clark, Lewisite, Lost, Sarin, Soman, Tabun or the like in soil, liquids and gases, wherein an electrode configuration having at least one working electrode and an opposing electrode is brought into contact with the substance to be examined via an electrolyte and a voltage is applied to the working electrode whose value is increased and decreased within a predetermined measuring time at least once and having the substantially same beginning and end values, wherein the strength of the current during this at least one measurement cycle is determined in dependence on the applied voltage. The invention also concerns a device for the detection of not easily volatized substances, in particular of nitrotoluene, trinitrotoluene, dinitrotoluene or derivatives of nitrotoluenes as well as of chemical warfare materials such as Clark, Lewisite, Lost, Sarin, Soman, Tabun or the like in soil, liquids and gases having a sensor element comprising at least one working and one opposing electrode which are connected to a regulated voltage source and which come in contact with an electrolyte.
Potentiostatic (amperometric) measuring systems are usually used, e.g. to detect gaseous substances which are not easily volatized, wherein a potential is applied to a substantially porous, large electrode. Electrochemical oxidation or reduction leads to the quantitative detection of the substance present via the amount of current flow. The qualitative detection is thereby preferentially facilitated by gas chromatography or by high pressure liquid chromatography (HPLC) or other methods with which the mentioned electrochemical systems are utilized as detectors.
In the conventional electrochemical sensor systems for gas phase analysis, which can also be based on semiconductor technology, the electrodes are separated from the gas volume by means of a membrane or polymer layer, wherein the substance to be detected diffuses through a permeable membrane and is subsequently dissolved in the internal electrolytes which are in contact with the electrodes. The detection of the respective substance is thereby effected by means of electrochemical oxidation or reduction at the working electrode. As has already been mentioned, a fixed potential is applied to the working electrode.
These types of methods, e.g. gas chromatography, have the disadvantage of being time consuming and expensive. In the event that liquids or soil is investigated it is first necessary to collect samples and remove same to a predetermined analysis location.
In addition, most explosive charges are produced using trinitrotoluene (TNT) or with the addition of TNT for reasons of simplicity of production and processing. Since this type of explosive is also used for extortion or terrorist threats as well as attacks, the detection of this type of explosive, e.g. during luggage and passenger checks for air travel, is an important task in order to prevent danger to human life. Also when searching for plastic encased mines, rapid and uncomplicated detection is imperative. TNT utilized for explosive charges or for the production of mines is a solid which releases small amounts of vapor to the surrounding environment.
In order to detect this type of contamination in soil, liquids and gases, EP-A-O 665 431 has already proposed a device working on the basis of cyclic voltammetry. This device has a sensor element comprising three electrodes, a working electrode, a reference electrode as well as an opposing electrode and having a regulated voltage source. A voltage is applied to the working electrode, the cyclic reduction and increase of which accompanied by simultaneous measurement of the current flow in dependence on the applied voltage, produces so-called voltammograms, obtained in the form of voltage-current curves. The voltage values on the respective working electrodes during the occurrence of cathode or anode current peaks for reduction and oxidation of the redox pairs represent a quantitative measurement for the contamination present. The quotient between the current strength of the anode current maximum and the measured current in the so-called double layer region leads to the determination of the TNT content. The above mentioned device can determine the TNT content or that of other substances per se, however the sensitivity is insufficient in the event of very small vapor pressures. In particular, a determination of substances in the gas phase is not possible with this device, since the determination of gas phase substances requires use of a membrane by means of which a flow-through cell is separated from the electrolyte solution.
It is therefore the underlying purpose of the invention to further improve a method as well as a device of the above mentioned kind in such a fashion that electrochemically reactive substances can be detected even in very small concentrations and/or vapor pressures and in a qualitatively as well as a quantitatively reliable manner.
This purpose is achieved in accordance with the invention by means of a method of the above mentioned kind in that a plurality of measurement cycles are carried out and the differences of current strengths of sequential measurement cycles are determined and, in the event that a cathode peak maximum occurs, the associated voltage value is determined, held constant and the current strength is determined. A device in accordance with the invention is distinguished in that the electrolyte is applied in the form of a thin layer onto the end surface of a sensor element coming in contact with the substance under investigation.
It has turned out that the cyclic voltammogram, for a plurality of cycles of the measurement cycle, stabilizes itself in such a fashion that reduction leads to a characteristic maximum (cathode current peak). Determination of the difference of the measured current values allows the occurrence of such a characteristic maximum to be determined in an optimal and rapid fashion. In accordance with the invention, when a cathode current peak occurs, the associated voltage value is determined, held constant and the current strength is then determined in a potentiostatic manner over time up to saturation. In this manner, the sensitivity is increased and the time for detection reduced. The qualitative detection is done by means of the peak potential determination and the quantitative detection by means of the potentiostatic measurement of the current height. In this manner, the response time is substantially improved. For example, response times of less than 0.5 seconds can be achieved.
The device in accordance with the invention provides that the requisite electrolyte be applied in the form of a thin layer on the end of the sensor element and not, as was conventional, by submerging the sensor element into an electrolyte. The thickness of the electrolyte layer influences the detection time. The method as well as the device in accordance with the invention thereby facilitate reliable detection of the most differing of not easily volatized substances even in small concentrations. The optimal achievable potential value with which a cathode current peak (negative sign) occurs is approached in a stepwise fashion using the method in accordance with the invention.
In order to prevent the measuring time from increasing unnecessarily, one measurement cycle spans, at most, the potential region between hydrogen and oxygen development.
The extracted or determined values with respect to current height, differences and potential can be output optically or acoustically. For example, an acoustical signal can change in dependence on the determined concentration of the respective substance. This is particularly advantageous when approaching a mine due to the associated increased concentration of TNT.
If the measuring medium, under all conditions, remains constant during passage through the measuring cycle, this leads to no change in the potential of the opposing or of a reference electrode respectively, when the latter is utilized. The potential associated with the cathode current maximum can correspondingly be used for detection of the subsequent TNT reduction. If however the measuring medium changes or the electrolyte composition changes with time this leads to a displacement of the potential. For this reason, instead of using the absolute potential at the cathode current maximum to determine the substance, a further improvement provides for use of the potential difference of the voltage values or potentials between an anode current maximum signifying an oxygen reduction and the cathode current maximum. It has been shown that this potential difference between the cathode current maximum and the oxygen reduction peak remains constant for the corresponding substance over a plurality of measurements. Measurements have shown that for 2.4-TNT and 2.6-TNT, a difference of approximate 840 mV is present. There is solely a dependence on the corresponding solvent and on the electrolyte utilized. 3.4-TNT has a different quantity of 950 mV. For 2-amino-5-nitrotoluene, measurements have revealed two reduction peaks of which one peak has a difference with respect to the oxygen reduction peak of approximately 860 mV and a second a difference of 480 mV. Knowledge of these potential differences thereby simplifies detection of the corresponding substance.
On order to perform continuous measurements, a substance under investigation is passed by the sensor element having the working electrode. In the event of a gas phase substance, this is preferentially passed through a flow-through cell. The gas phase substance is thereby preferentially pumped through the flow-through cell and passed by the sensor element. In this manner, even substances of low concentration and low equilibrium vapor pressure can be detected.
In order to be able to apply the electrolyte to the sensor elements in a simple and rapid fashion, same is preferentially a viscous fluid or a gel-like substance. Such an electrolyte can be applied rapidly and bonds in a reliable fashion to the end of the sensor element. Such a sensor element can also be introduced into a gas volume in a simple and rapid fashion to detect gaseous substances. Membranes are no longer needed to separate the gas volume from the electrolyte.
Electrolytes can, in addition to inorganic compounds (sulfuric acid, perchloric acid among others), also be mixtures made from inorganic and organic compounds (acetone/acid, ethanol/sulfuric acid, hexane-1-sulphonate/sulfuric acid among others) as well as pure organic compounds (acetone, ethanol among others). The choice of the respective electrolyte depends on the required sensitivity and selectivity when carrying out the method in accordance with the invention and therefore can be tailored to the substance to be detected.
A preferred embodiment provides that at least the working and/or opposing electrodes are microelectrodes. Preferentially they are ultra microelectrodes. In this manner, the measurement can be carried out with a simple two electrode configuration. The opposing electrode is simultaneously the reference electrode. This simple construction is possible since the potential on the opposing electrode remains nearly constant throughout the measuring cycle in consequence of the low current flowing at the microelectrodes.
The working electrode is preferentially substantially enclosed in a ring-shaped fashion by the opposing electrode. In this manner, a compact construction of the sensor element is nevertheless associated with a large surface for the opposing electrode as well as an even current distribution on the working electrode. In addition, the opposing electrode is thereby substantially non-polarizable in the electrolyte, since the surface of the opposing electrode in contact with the electrolyte is substantially larger than that of the working electrode.
The sensor elements can also be equipped with a plurality of working electrodes and/or a separate reference electrode can also be incorporated.
An improvement provides that electrodes be disposed insulated from each other within a measuring head. The electrolyte layer is then introduced on the lower side or end of the measuring head. The measuring head is preferentially made from glass or from a mould substance such as ARALDITE or the like. These substances could also serve as insulators between the electrodes.
The triangular shaped voltage/time dependence in cyclic voltammetry at the working electrode facilitates, in addition to the quantitative and quantitative information, additional monitoring of the surface of the sensor element or of the measuring head as well as self-calibration of the system.
An improvement provides that a flow-through probe be disposed below the sensor element or the measuring head. In this manner, gaseous substances can be easily continuously investigated. For example, the flow through cell can be bolted onto the sensor element. In this manner, the substances to be detected can be selectively and reliably measured. Improvements provide for a seal between the measuring head and the flow-through cell. A pumping device is preferentially provided in order to guide the gas past the measuring head. The membrane of prior art is no longer necessary in the device in accordance with the invention to analyze gaseous substances.
A further preferred configuration provides that the sensor element or the measuring head can be connected to a regulation and evaluation electronics. In this manner, the current values measured at the applied voltage values, the voltage values, and the differences of the current values can be analyzed and the corresponding concentrations of the material to be detected can then be indicated. An output unit to display concentrations is preferred so that same can be directly read during measurement. It is also possible to indicate the values not only optically but also acoustically, wherein e.g. an alarm unit for output of an alarm signal, in the event that a predetermined concentration is exceeded, can be provided for, wherein the acoustical signal can advantageously depend upon the concentration.
The method in accordance with the invention and the device in accordance with the invention thereby allow for the qualitative and quantitative detection of electrochemically reacting substances even at very small vapor pressures. The use of differing electrode materials and electrolytes in the measuring head or the sensor element can provide for a selective detection of differing substances. The sensitivity and selectivity of the measurement system which thereby result allow for applications in the most differing of regions, e.g. environmental protection as well as personnel protection in which the polluting chemical substances must be detected in low concentrations. For example, TNT can thereby be detected in the gaseous phase. In addition, the method and the device in accordance with the invention can be utilized not only e.g. to search for mines containing TNT but also for explosive charges and for the cleaning of contaminated surfaces in a reliable and rapid manner. In addition, a series of other substances e.g. chemical weapons such as Sarin, Clark I, Lewisite, Soman, Tabun as well as Lost etc. and drugs and substances having high vapor pressures can be detected.
Further advantages and features of the invention can be extracted from the claims and the subsequent description in which an embodiment is described in detail with reference to the drawing.